On the stereoselectivity of epoxide formation using dimethyloxosulphonium methylide. X-Ray structure of (5SR)-5-[(1RS)-1-methyl-2-oxacyclopropyl]pyrrolidin-2-one

As background to a proposed dendrobatid toxin 251 D synthesis, the stereoselectivity of epoxide formation from 5-acetylpyrrolidin-2-one (4) and dimethyloxosulphonium methylide was investigated. In THF under ‘salt-free’ conditions, the major epoxide product, selectivity 78 : 22, was (5SR)-5-[(1SR)-1-methyl-2-oxacyclopropyl] pyrrolidin-2-one (5), whereas addition of anhydrous ZnCl2 to the reaction mixture reversed the stereoselectivity to give epoxides (5) and (6) in the ratio 23 : 77. Configurations were assigned to these epoxides by comparison of n.m.r. spectra of the derived carbamates (15) and (16), and by an X-ray structure determination for epoxide (6). Related reactions are discussed.